Insulated embankment design techniques

ABSTRACT

Described are techniques and construction arrangements for providing insulated embankment foundation embodiments for liquid storage containers in &#39;&#39;&#39;&#39;arctic temperature&#39;&#39;&#39;&#39; environments; this including, in specific embodiments, techniques for determining optimum the thickness and other characteristics of associated insulation, as well as for subjacent gravel transition layers, plus other techniques such as pre-conditioning and refreeze schedules related to particular embankment design and service conditions.

[ 51 Nov. 12, 1974 1. 1 INSULATED EMBANKMENT DESIGN TECHNIQUESInventors: Glenn R. Burt, Deer Park, Tex.;

Albert C. Condo, Newton Sq., Pa.; George R. Knight, College, Alaska [73]Assignee: Atlantic Richfield Company, New

York, NY.

[22 Filed: July 10, 1972 [21] Appl. No.: 270,359

152] U.S. Cl 61/.5, 61/36 A, 61/50 {51] Int. Cl .l. ElT2d 3/UO [58]Field of Search 166/D1G. 1; 61/36 A, 50,

156] References Cited UNlTED STATES PATENTS 3,768.547 10/1973 Bestl66/D1G. 1

3.667.237 6/1972 Dougan 61/36 A 3,602,323 8/1971 Scliuh 61/50 3.27933410/1966 Quartararo 404/31 3.135.097 6/1964 Scheinberg 61/36 A 3.524.3208/1970 Turzillo 61/38 OTHER PUBLICATIONS Permafrost by S. M. Muller, p.153, 177 J. W.

Edwards. lnc. Ann Arbor, Michigan, 1947.

Arctic Construction" US. Army Engineer School CNC.02212, Oct. 1961.

Better Building Bulletin No. 5, Permafrost and Buildings" Sept. 1955,National Research Council. Permafrost and Related Engineering Problems"Endeavour, Vol. XXlll No. 89, May 1964.

Permafrost and Related Engineering Problems," Muller, pages 98-103,123-126, T. W. Edwards, lnc. Ann Arbor, 1947. I

Primary Examiner-Robert R. Mackey Assistant Examiner-Alex GroszAttorney, Agent, or Firm-Coleman R. Reap [57] ABSTRACT Described aretechniques and construction arrangements for providing insulatedembankment foundation embodiments for liquid storage containers inarctic temperature" environments; this including, in specificembodiments, techniques for determining optimum the thickness and othercharacteristics of associated insulation, as well as for subjacentgravel transition layers, plus other techniques such as pre-conditioningand refreeze schedules related to particular embankment design andservice conditions.

12 Claims, 17 Drawing Figures REF. f

SUB.

PAIENIEDHBY 12 m 11846389 sum 1 or e PB-L I T-W P T-V jP-BH P-B ZREI?ZREE SUB.

TYPICAL RESERVOIR CROSS SECTION FIGURE I MEAN DAILY AIR TEMPERATURES1ST. YR.

THAW INDEX 1 III NQI=692F DAYS I I W /W fl II II A III JAN FEB MAR APRMAY JUN JUL AUG SEP OCT NOV DEC FIGURE 2 M AN DAILY AIR TEMPERATURES2ND. YR.

THAW INDEX TIN l I 8,895F DAYS= Fl IN" II II FIGURE 3 JAN FEB MAR APRMAY JUN JUL AUG SEP OCT NOV DEC PMENTEDIIIIV I 2 m4 3i846l989 sum 2 or sWATER DEPTH IARIES 0 TO lO FT T-iw ARco FOAM' THICKNESS 0T0 em I W 0, 5GRAVEL DIVIDED INTO 5 ONE-FOOT .54 5 3 LAYERS WITH VARIATION IN wATERCONTEN'II ACTIVE LAYER L5 FT IO HIGH ICE CONTENT SILTY PERMAFROST &

MEDIUM ICE CONTENT SILTY PERMAFROST TO DEPTH OF 50 FT.

SOIL PROFILE (SITE NO. I)

FIGURE 4 MEASURED 8 CALCULATED WHIPLASH CURVE SITE No.2 DAY NO. I

TEMPERATURE F DEPTH o FEET o MEASURED CALCULATED FIGURE 5 MEASURED &CALCULATED WHIPLASH CURVE SITE NO.2J DAY NO.I 4- I73 TEMPERATURE F I0 I?29 2 5 3 0 3 5 40 DEPTH FEET 3 o MEASURED CALCULATED FIGURE 6 PAIENTEWWIM 31846389 sum am a MEASURED 8| CALCULATED WHIPLASH CURVE M TEMPERATUROFIO 2O 25 35 40 O l l l J DEPTH FEET I 0O g o MEASURED CALCULATED FIGURE7 IST THAW INDEX IST FREEZING INDEX ZNDTHAW INDEX CALCULATED TEMPS AT:EASURED TEMPS AT; 4 4.25 0 IO 2385 7.25 0 I I425 o 33.0 33.25 0

J FIMIAIMIJIJ IAISIOINIDIJIFIMIAIMIJIJlAlSlOlNlDl FlRST YEAR SECOND YEARSITE NC). 2 CALCULATED 8 MEASURED TEMPERATURES FIGURE 8 4-+ NEAR SURFACEOF TANK (QG'DEPTH) O-QNEAR BOTTOM OF TANK (9.9 DEPTH) so A l\ TEMP. (F)

I4l82328 3 8 l3l82328 27 I2I72227l 6 ll l6 2| 2630 JUNE JULY AUGUSTSEPT.

FIGURE 9 YEAR N02 PATENTEDNUY 12 I974 SIEU FT OF THAW FT. OF THAWUNINSULATED SERVICE FACILlTY-TANK FILLING l OCTOBER ALTERNATINGSOLUTIONS-UNINSULATED FACILITY FIGURE l2 INSULATED EMBANKMENT DESIGNTECHNIQUES BACKGROUND OF THE INVENTION Workers in the art of coldweather construction are commonly confronted with the task of providinggravel embankments, or like foundations, for roadways, stortions, etc.,in arctic temperature regions such as the Alaskan North Slope. Aparticularly vexing problem associated with such construction is how toprovide an embankment foundation for a'heated water reservoir. Such astorage facility may be contemplated for providing a continuous potablesupply of water in an unfrozen liquid state to various man-supportfacilities, etc., in the arctic. The water must obviously be kept warmto avoid freezing and keep it liquid and available for humanconsumption, for chemical processing and especially for fire fightingpurposes.

It was, at first, believed that if a worker analysed theinterrelationship of temperature, soil depth and time for such areservoir installation, it would be possible to apply a somewhatconventional, three-dimensional analysis and calculations and therebyevolve, for instance, a plot of temperature shift vs. driving thermalconditions doing so on a time dependency basis for various points aroundthe base of such a structure, and to, at length, generate an ultimatesteady-state solution (for infinite time). With such a'technique a thawbulbs could be delineated under a reservoir of the type mentioned,situated at varying depths thereunder and emanating from the center ofthe reservoir base to its corners (or generally to its outer periphery).Such a study will also suggest that the thaw locus (the subsurfacelocation of the thaw front or 32F. isotherm) will quickly drop undersuch aheated reservoir. For instance, in the case of a reservoir tankfacility of particular kind (see Examples 1 and 2 described below) thethaw front will have dropped to about 5 feet below grade afterapproximately 200 days. Thawing would proceed further after 1 year toreach approximately feet (upper 5 feet of permafrost completely thawed)and a depth of about feet reached after about 700 days with a relativelysteady state or equilibrium taking place at approximately feet below thecenter of the tank, assuming no settlement.

Such analysis, of course, would ignore latent heat factors, would assumethat approximately 200 days after a summer-filling of this tank (orapproximately in February) the active layer of tundra would be thawed(beneath the gravel embankment under the tank). The total thaw of theaverage active layer could be expected to result in approximately 4 to'6inches of surface subsidence. After about 1 year or more, ice wedgesmelt and a surface settlement of 4 to 10 feet is possible where thewedge had existed; whereas in regions overlying the center of typicalArctic polygons like settlement would be somewhat less, perhaps 2 to 3feet.

Now, obviously, such differential settlement is not acceptable; it canreadily lead to degradation and destruction of the embankment and thetank facility and support equipment located thereon, with possiblerupture of the tank, etc. Major maintenance and regrading operationswould also be required following each summer of such service, entirelyaside from the problem of how to deal with this differential settlement.

' age sites, building constructions and reservoir founda- The presentinvention is intended to provide a better solution to this and relatedproblems. Accordingly and to meet the foregoing problems, we havedeveloped techniques, according to various features of this invention,that involve such expedients as introducing a prescribed artificial turfunder a heat-emitting facility (like this mentioned reservoir) which,together with a superimposed layer of insulation, makes it possible topredict an extended, definite service time for the facility,togetherwith a schedule for cooling the overall facility in a prescribedmanner (before service and in some cases after service periods or duringprescribed refreeze periods). Another feature involves indicating howprescribed measurable environmental factors such as the thermal historyof the site and the moisture and other thermal parameters of embankmentmaterials may be used according to novel calculation methods plus anovel design technique adopted to establish the parameters for suchembankment construction. More particularly, this involves specifyingprescribed embankment construction for supporting a given heatedfacilitywherein the thickness of the aforementioned artificial turf is set inkeeping with a prescribed service life. In particular instances thisalso involves operating the tank facility according to a prescribedservice refreeze schedule, whereby improved, continuous overalloperation is achieved by alternating service time and refreeze time.Other features will be apparent to those skilled'in the art from thefollowing description of the particular embodiments of this invention.

Accordingly, it is an object of the invention to pro vide a solution tothe aforementioned problems and in general to provide the features ofnovelty and advantage described herein. Another objective is to providethe foregoing in an arrangement including prescribed composite insulatedembankments using prescribed techniques for construction and fordetermining the characteristics of the artificial turf material and itsthickness. A related object is to provide the foregoing so as to afforda prescribed thermal impedance whereby to retard movement of the thawlocus associated with the facility for a prescribed extended period oftime. Another object is to provide the foregoing in keeping with aprescribed schedule of cooling periods and service life. A furtherobject is to provide the foregoing using a prescribed design techniquebased upon calculations from known, available environmental thermalparameters and material characteristics. Yet another object of thisinvention is to provide a design method for constructing insulatedfoundation pads on permafrost whereby to support a heated facility.(e.g., being kept at a constant service temperature, above freezing.) Astill further object is to provide such design techniques wherein thefacility operates at a constant temperature above freezing and yet willtransfer its and defines, a prescribed service period for the facilods,the means by which such thermal restoration and dissipation ofgeothermal heat plus other heat accumulated in the pad during service isremoved at a predetermined rate, dependent on design constraints. Thetime lapse from elevation of the thaw front (from the active layertundra surface to its original position as constructed) to just-prior"to being placed in service is referred to as the refreeze period. Thisrefreeze period is a non-productive period during which the facility isnot operative (as a reservoir). Still another object is to provide,within such design constraints, a structural facility which so minimizesthe length of this refreeze period as to maximize the ratio of servicetime to refreeze time. I

The invention and means by which the objects of this invention areaccomplished will become clear in the Examples to be hereafterdescribed. Also, the various objects and advantages of the inventionwill be fully understood from reference to the following detaileddescription of the preferred embodiments of the invention when taken inlight of the accompanying drawings, wherein is indicated a typicalembankment construction for a cold environment together with variousmodes of construction and associated curves indicating the effectsthereof.

IN THE DRAWINGS:

FIG. 1 is an idealized elevational section'of such an embankmentembodiment designed as a foundation for a heated reservoir tank situatedupon a site characterized by permafrost with certain items beingonlyfragmentarily and/or schematically indicated for simplicity;

. temperatures (calculated and measured) vs. depth at a second site,similar to that of FIG. 1, as of a certain date, then 6 months, and 18months thereafter, respectively;

FIG. 8 comprises curves indicating temperature fluctuations (calculatedand measured) at various subsurface depths on the site of FIG. 1 over a2 year period;

FIG. 9 shows curves plotting temperature fluctuations with time at twolocations in the reservoir tank of FIG. 1;

FIG. 10 is a plot of the locus of the thaw front with time in anembodiment like that of FIG. 1 in the face of certain different testconditions, including placement of tank, then filling with heated water;while FIG. 11 is a similar plot, with site cooling and a tank fillingtaking place over different seasonal periods however; while FIG. 12 isalike plot for other site conditions; and FIG. 13 is a like plot forfurther, more representative site conditions as a function of foaminsulation thickness.

FIG. 14 shows the shift in thaw locus in time for various insulationthicknesses in an embodiment like that of FIG. 1;

FIG. 15 indicates the rate of temperature rise over extended time 50feet below the embodiment of FIG. 1 for various insulation thicknesses;

FIG. 16 shows two families of curves; each family indicates how theprofile of subsurface temperatures relative to embodiments as in FIG. 1shifts with cooling time, one family with insulation; the other withoutit; and

FIG. 17 shows plots of thaw depth vs. service time for embodiments as inFIG. 1 having various thicknesses of insulation; together with a plot(Y-R in phantom) of insulation thickness vs. refreeze time.

Reference will first be made to the construction of FIG. 1 to indicatethe context of a typical (cf.EXAM- PLE I, however it being understoodthat no insulation Ti is employed) problem circumstance and as a meansof developing, explaining and applying various improvement features ofnovelty.

EXAMPLE 1 The Problem A service foundation pad P, as seen in FIG. 1, wasconstructed atop the tundra-subsoil substrate (terrain sub) at alocation (site No. 1) on the arctic plain. This was done by compactingriver bank gravel hauled from a river borrow pit nearby. The gravelthickness (P-h) above the tundra was 5 feet. Pad P is surrounded on allfour sides by a gravel berm PB to a maximum height (P-BH) of 18 feet andwith a 3 on I shoulder and an overall (PB-L) of 201 feet from toe to toe(as shown in the cross-sectional view in FIG. 1). The space betweenberms, and above the 5 foot base forms an inverted trapezoid whichdefines a space for coating a one million gallon capacity water storagetank T, only the walls TW thereof being shown and including an innerlayer of fiber reinforced vinyl Tv. The top side surface of the watertank may preferably include a thermal insulation cap, such as a 2 inchlayer flexible polyurethane foam (not shown), which retards the loss ofheat from the waterfall W to the cold ambient air. The stored water(fill W) in the reservoir tank is recirculated thru a heat exchanger(not shown; coventional) thru an outlet system (only one conduit ofwhich, 14, being shown, and this fragmentarily) at a rate to maintain aconstant temperature of 35F, at the exchanger. Thermistors connected todata aquisition equipment (neither shown) were installed at various testdepths (e.g., see test sites 1) down to 15.5 feet at the center, beneathoutlet 14, and beneath the berm. The facility was constructed in thefall, allowed to freeze, and subsequently filled the next January.Shortly thereafter, the reservoir failed due to thaw beneath an outletconduit (e.g. l4) and consequent subsidence (of the melting permafrostunderneath) and collapse of the conduit.

EXAMPLE 2 Deriving SST Curves FIGS. 5-8

A set of thermistors were installed in the 5 foot gravel embankment overthe tundra at site No. l to several embankment-sampling depths of 4.25,7.25, 14,25 and 33.25 feet below the gravel surface (i.e. all except the4.25 foot site lying under the top surface of the terrain i.e., belowreference grade REF in FIGS. 1 and 4). During two successive 12 monthperiods, ambient air temperatures were recorded daily and the mean dailyair temperatures (simple arithmetic mean calculated) were plotted versustime (see FIGS. 2 and 3, respectively). The soil profile for this site(8-1) was determined from various soils samples and averaged. Theaveraged soils profile for site No. I is shown in FIG. 4, were heatedwater fill W, embankment cross-section P-B including layers having watercontent W,W

(varying from 5 at top to 14 percent at bottom), ground surface locusREF and subsurface strata SUB are as indicated for FIG. I. ARCO foaminsulation layer T i will for the present be ignored and assumednonexistent. Subsurface strata SUB comprises a top Active Layer(aforementioned) averaging about 18 inches of vegetation, rocks, siltand like tunda-soil material. Layer AL will be subject to thaw undernative ambient conditions, at least occasionally (to fill depth). Underlayer AL is a layer U-P, of high moisture (ice) content, silty soil;assumed perpetually frozen (permafrost) under native ambient conditions;with a second permafrost layer L-P; under layer UP;, however beingtypically thicker (about 50 vs. about feet) and of lower moisturecontent. Table I is a tabulation of the properties to be assumed for thematerials indicated for the soils profile of FIG. 4.

The above-freezing ambient air temperatures indicated in FIGS. 2 and 3reflect the heat-engine or driving force applying heat at a certain rateto any soil surface which in turn acts to transfer the heat flow tosubsurface depths.

Ambient air temperature fluctuations can be translated into soil-surfacetemperatures fluctuations and to sub-soil temperatures at variousdepths. Mathematical techniques have been developed which provide ameans for effecting this translation; however, these are limited to asomewhat-idealized mean annual sinusoidal air temperature fluctuationandto simplified soil strate conditions; and accordingly deviatesomewhat from actual temperature measurements taken-at varioustest-depths in the soil.

TABLET? temperatures are also recorded for Day No. 1 and N- factored toestablish the mean daily soil surface temperature. A whiplash curveshowing the variation of temperature with subsoil depth is thus derived.

FIG. 5 shows such a whiplash curve for Day No. 1 taken at site No. 2removed a distance from site No. l but similar thereto for the presentpurposes. FIG. 6 shows the same curve taken about 6 months later andFIG. 7 shows the same curve for conditions about 18 months later. Thespine (SP) of these curves (below 30 feet) will be seen to remainrelatively the same (vertical), indicating constant temperature (closeto 15F.), while the upper soil or whip sections oscillate back andforth, indicating that the temperatures so 05- cillate with the seasons,changing more radically the closer to the surface one gets (thewhip-action). Referring to FIGS. 5, 6 and 7 it has been seen in practicethat the theoretical sub-soil temperatures which were calculated at 0, 6and 18 months (after Day No. 1, using the referenced finite differencingprogram and ambient air temperature driving conditions from airtemperature records for that day) correlate well with measured values.That is, actual thermocouple measurements of temperature at these depthsover the same period yields curves that fit closely to the indicatedtheoretical curves. The validity and accuracy of this FiniteDifferencing technique are undoubted now. FIG. 8 shows the sinusoidaltemperature fluctuations over a 2 year period as measured and ascalculated for several depths in and below a 5 foot embankment. Thiscontinuous 2- year set of subsurface soil temperatures (SSTs) asinfluenced by air temperature correlates well with the curves of FIGS.5, 6 and 7.

PROPERTIES OF MATERIALS IN SOILS PROFILE OF FIG. 4

Latent Dry Moisture Thermal Vol.1-1eat Heat of Thermal Density ContentConductivity Capacity Fusion Diffusivity pcf BTU/fthrF. BTUlcuftFBTU/cuft sq ft/day ARCO- FOAM 2.0 0.0 0.0125 1.0 0.0 0.003 Gravel 120.05.0 1.08 24.9 864.0 1.04 Gravel 120.0 7.0 1.30 26.8 1210.0 1.17 Gravel120.0 9.0 1.50 38.5 1555.0 1.26 Gravel 120.0 12.0 1.78 31.2 2074.0 1.37Gravel 120.0 14.0 1.96 33.0 2419.0 1.43 Act Layer 90.0 30.0 0.97 35.53888.0 0.66 Silt 35.0 142.0 1.28 30.6 7100.0 1.00 Silt 56.0 72.0 1.2830.2 5930.0 1.02 Water 62.4 100.0 0.35 62.4 8990.0 0.14 Ice 57.0 100.01.28 0.98

A finite differencing computation technique will be described wherebythe soil composition and thermal properties for any subject strata'areemployed along with (reported) actual daily temperature data toestablish resultant subsoil temperatureprofiles verified by establishinga direct correlation between theoretical and measured values of sub-soiltemperature (driven by ambient air temperatures) these-profiles shiftingof course with seasonal thermal changes.

First, it is desirable to measure ground temperature, at variousselected depths beneath the 5 foot gravel pad, these being recorded (orcomputed) to be plotted as a function of ground surface temperatureconditions on a selected starting day (Day No. 1). Ambient air On thebasis of the foregoing fand as further explained, it will be seen thatthe finite differencing calculation method can be employed to calculatesubsurface temperatures for any locale in the Arctic or sub-Arctic wherea relatively one-dimensional heat flow can be anticipated withreasonably level terrain and at a point not significantly influenced byan adjacent lake, stream or like thermal anomaly.

The input ambient'air temperature data will be correlated with the airtemperature reports from the nearest weather station that provides suchon a current basis. A conversion factor (the N-factor) computations canthus be developed to convert such baseline (reference) weather data forthe (nearby) site in question.

Appropriate soil conditions for the specific site must of course bedetermined, establishing the site and composition of sub-soil layers andtheir properties. If ground surface temperature data is not available,it may be derived by known calculations as a function of the sinusoida]air temperature fluctuations at the site. This will constitute the(quasi-steady-state) heat in-flow to the subsoil.

With this initial data and calculations, one can employ conventionalweather information over the prior -20 years and thus derive a generalapproximation of weather data for the coming 2-20 years, includingresponsive fluctuations in ground temperatures. This, in turn, will helpto provide the necessary information required for forming the subjectnovel design for embankments over permafrost. More especially, aprobability basis will be determined to indicate the extent andfrequency. of deviations from given mean temperatures. Furtherparticulars will appear from the following Examples.

EXAMPLE 3 Heated Tank; Optimize Fill-Time (FIG.

The tank facility of Example 1 is analyzed to establish tank watertemperature, and to determine an optimum time of year for filling thetank. Analysis of the facility and ambient thermal factors indicatesthat, assuming the thaw conditions of year No. 2 (FIGS. 3 and 8) and atank maintained reasonably full through the summer period and assuming10 feet of uncirculating water and no heating (see FIG. 9), a largetemperature gradient from top to bottom of the tank will result. FIG. 9shows that at the bottom of the tank, water temperature was relativelyconstant, 35F. Such a bottom temperature" is acceptable; beingaccompanied by an upper tank" temperature of 50F. and more. Ifcirculation is maintained during the four month summers, the tank waterwould have a mean temperature of 39F. (throughout), resulting in alarger bottom-of-tank thaw index (Tl This clearly indicates that watercirculation is not advisable during summer. Accordingly, it is preferredthat the circulation system be selectively controlled by a relay systemthat permits circulation during the summer only when bottom-water is at,or below, 34F.

The best time to fill the tank in reference to the summer period can bearrived at by a thawing index comparison. A tank filled in the spring ofthe year and allowed to remain quiescent so that the temperature doesnot exceed 35F. will produce a bottom thaw index on the gravel ofapproximately 700F-days (see below). This compares with an air thawindex approximately twice as great. The foregoing was arrived at bycomparative analysis of the following tank conditions:

I. Tank filled on the first of May and water temperature inside of tankheld at 35F.

ll. Tank left empty after the first of May with a 2-inch flexible foaminsulation lying on the gravel surface.

Analysis of the empty tank (Case I) showed approximately 3 feet of thawinto the gravel beneath the tank by October lst; whereas the tank filledon May 1st (Case II) would have only 2 feet of thaw into the gravel.These analyses (elaborated below re Example 5) clearly indicate thatfilling should take place as early as practical in the spring, if notbefore (preferably fall or winter; and after refreeze in any event).

Now, in brief recapitulation, workers in this art will recognize thefollowing novel features as derived and exemplified above. Gravelthickness P-h in Examples 1-3 (FIGS. 1 and 4) is, in reality, arrangedto provide a prescribed artificial active layer (or pseudo turf) underthe heat-emitting tank facility which, together with a superimposedlayer of insulation, can so retard the thaw front (under specifiedambient conditions), to allow a prescribed service time" to bescheduled, together with a following recuperative cooling period ifneeded. Using prescribed measurable environmental factors, such as thethermal history of the site and the moisture and other thermalparameters of construction materials, workers in the art may availthemselves of a novel improved design and techniques for embankmentconstruction.

EXAMPLE 4-A is uninsulated or insulated under tank T. Here, as in Ex-'ample l, the basic soils profile of FIG. 4 and the prop,- erties of eachstratum tabulated in Table I will be assumed; however, with inclusion ofinsulation layer T-i.

The cross-sectional view of the insulated foundation P for the heatedtank facility shown in FIG. 1 indicates the 18 foot berms, P-B, the 5foot gravel pad, Ph, heated water reservoir, W, in relation to eachother and also to a layer of thermal insulation T-i (not assumed inprior Examples). Insulation T-i preferably comprises an ARCOFOAM-lpolyurethane system constituted and installed as described in copendingcommonlyassigned US. Pat. application 227,664, filed Feb. 18, 1972,entitled Stabilizing Arctic Ground Cover, by Albert C. Condo and JosephE. Neubauer.

This ARCOFOAM-l system (including isocyanate ISO-1A plus polyol OL-IB)is spray-applied on the gravel pad as indicated in FIG. 1. Itsproperties and characteristics will be as summarized in Table I-A asfollows:

TABLE I-A ARCOFOAM-l" PROPERTIES Compression strength (psi) at yieldCompression at yield Dcnsity(pcf) K-factor Closed Cells 8 Open Cells('70) Cell Walls ARCOFOAM l foam provides a high ratio of compressionstrength to density.

Preferably, a moisture barrier precoat (subcoat) is used with ARCOFOAM 1(thereunder) such as Ar cote Weathercote as described in the referencedapplication.

The aforementioned Finite Differencing method is best understood andexplained in terms of known initial conditions and known subsurfaceproperties. To this end, various test conditions were postulated for thefacility of Ex. 1 for selected test periods and in various sequences.(Table II below). To determine (calculate) their thermal effects asrepresented by the resultant position of the thaw-front, in or beneaththe gravel embankment (FIGS. 10-13 as follows); the plot of thaw locus"thus derived indicated (expectedly) that the effect of each testcondition was (somewhat) dependent upon the effects of the prior testcondition. Here it should be assumed that all test conditions relateback to the described inadequacies of the service facility of Example 1.Also, the Finite Differencing used will be understood as allowing for anormal variation in moisture gradient within the gravel and for normalvariations in the properties of the foundation soils.

The test conditions invoked are summarized in Table II below. Analysiswas run on these and the aforementioned sets of subsoil temperatureswere generated over the indicated time and seasons for the site.

TABLE II 1. Air temperature on feet gravel (moisture content, w:14percent).

2. Air temperatures on 8 inches ARCOFOAM over 5 feet gravel (w:l4%).

3. 35F on 6 inches ARCOFOAM over 5 feet gravel (w:variable).

4. Air temperatures on 6 inches ARCOFOAM over 5 feet gravel (w:l4%).

350F. on 4 inches ARCOFOAM over 5 feet gravel 6. Air temperatures on 2inches ARCOFOAM over 5 feet gravel (w:l4%).

35F. on 5 feet gravel (w:l4%).

8. Air temperatures on 5 feet gravel (w:variable).

9. Air temperatures on 2 inches ARCOFOAM over 5 feet gravel(w:variable).

10. 35F. on 5 feet gravel (w:variable).

11. Air temperatures on 2 inches flexible foam over 5 feet gravel(w:variable).

12. Air temperatures on 6 inches ARCOFOAM over 5 feet gravel(w:variable).

13. 35F. on 4 inches ARCOFOAM over 5 feet gravel (w:variable).

14. Air temperatures on 4 inches ARCOFOAM over 5 feet gravel(w:variable).

15. 35F. on 2 inches ARCOFOAM over 5 feet gravel (w:variable).

16. Air temperatures on 2 inches flexible foam, 10 feet water, 2 inchesARCOFOAM over 5 feet gravel (w:variable).

17. -F. on 2 inches flexible foam, 2 inches AR- COFOAM over 5 feetgravel (w:variable).

By way of further explaining the calculation sequences (runs) of TableII, application of Test Conditions Nos. 1, 2 and 3 will now bedescribed, these being invoked, successively, for certain respectivetime periods. The pad P of FIG. 1 is initially assumed to be affectedonly by ambient air temperatures acting on the 5 foot gravel thicknesswith an average moisture content of 14 percent during the periodstarting on the first of February and ending the first of August. Thisis fol lowed by test-condition No. 2 wherein 6 inches of AR- COFOAM and2 inches of flexible foam over 5 feet of gravel with 14 percent moisturecontent is left in situ until the first of October; this, in turn, wasfollowed by test condition No. 3 whereby tank T with 35F. water isassumed placed upon 6 inches of ARCOFOAM, overlying the 5 feet of gravel(variable moisture content) for an additional 5 years. The results ofthis (No l, 2, 3) sequence indicated specific subsoil temperatureprofile sets, varying with time, along with the average temperaturedegree which, of course, varied seasonably from the site. This was usedto plot thaw front location; for example, in FIG. .13, treated below;the thaw iso-therm location is picked out of each tion thickness as wellas for zero thickness.

FIG. 10 indicates the effects of different successive test conditions ona test embankment facility as a function of thaw locus; i.e., howseasonal weather (thermal) changes at the test site (site No. 1) andcertain service conditions (tank installation, then filling with heatedwater, kept at 35F.) can affect the location of the thaw-front (i.e.,lowermost location of 32 isotherm). Here, conditions 8, 11 and (amodification of) No. 10, obtain for the indicated seasonal periods. Thefacility of Example 1 will be assumed as employed here, with a 5 footpad (outer tank, on frozen substrate same gravel type, having variablemoisture content per FIG. 4) on which is situated acollapsible-inflatible pillow tank having walls of 2 inch flexible foaminsulation and a vinyl insert outside, later being substantially filledwith water kept at 35F.

FIG. 11 assumes the same test situation except that a longer anddifferent air-only period obtains (vs. condition No. 8), a differentempty-tank period obtains (vs. condition No. 11) and tank filling(start-ofservice) occurs 3 months later (on January I). The prolongedeffect is that, as a result of a cooler, moreeffective pre-conditionperiod (analogous to amore effective refreeze period), the curve(thaw-locus) is kept up (higher on the embankment) for a longer period(e.g., takes about 3 months longer to reach 5 feet depth).

As aforementioned, FIG. 12, like FIGS. 10, and 11, plots the shift inthaw-locus with different test conditions (cf Table II) and in variousrespective seasonal periods doing so for a pad which is effectivelynoninsulated.

FIG. 13 plots the same thing for a more specific and practical set oftest conditions (tank of FIGS. 10, 11 used here also), simulating thepreconditioning and use of the facility as in Example 2 etc. withvarious thicknesses of foam insulation (and zero foam as a baselinereference). Consideration of FIG. 13 shows that application of 2 or 4inches of foam gives a marked, surprising improvement (e.g., extendingthaw-time) over the zero-insulation case; and that going to yet thickerinsulation (6 inch foam) brings one closer to a point of diminishingreturns as well as inducing a surprisingly-long initial cooldown time.The latter suggests that the rectitherm effect aforementioned is at workand that a consequent loss of refreeze effectiveness will result(further discussed below, see FIG. 18 and discussion).

EXAMPLE 4-B A careful appraisal was made to establish the operatinglimitations of the tank as affected by the uninsulated 5 feet of gravelwith 35F. water as in Examples 1 and 4-A. This appraisal was made todetermine how the existing tank of Example 1 could be returned toservice. Here it is assumed that the tank'is to be repaired and placedon the embankment ready for filling on (September 1 of filling year No.1, then being filled the next month (on October 1) this compared withfilling on January 1 of the next year. The outputs from the FiniteDifference analysis of Example 4 showed that 18-24 months (see FIG. 13;no insulation, 4-5 feet thaw; also FIG. 11) would be required in thecase of the January filling for thaw to take place through the gravel toa point where addition thawing would create excessive settlement.

If the embankment supporting the tank is to be operated withoutinsulation, it thus appears that the tank must be left empty tocool-down for a reasonable time each winter 1-3 months) sufficient torefreeze the embankment. The analysis of the necessary refreeze periodassumed that the average mean temperature from November 1 through April1 will be less than lF., (based on previously given known winter datafor this site No. 1). This appraisal showed that allowing the tank toremain in place, and cooling without any water in it for approximately 3months would result in the subjacent temperature returning very close tothat of the native condition (i.e., where no such reservoir operationexists to affect it).

EXAMPLE 4-C The specific location of an insulation layer within theservice embankment causes slight variations in the thermal design. Forthe specific case of the reservoir of Examples 1 and 4, it was assumedan insulation layer would be placed directly on the gravel and up theside slopes as per FIG. 1. Analyses were performed on the assumption of2, 4 and 6 inches of ARCOFOAM in Example 4A to establish the time rateof thaw beneath the insulation. These analyses assumed that theinsulation and tank would be placed in refreeze condition September I(RD-Dry zero). The analyses are based on fillings on Oct. 1 or January 1(FIGS. 10, 11 respectively). The studies assume maintenance of the 35F.temperature for the number of years to result in a maximum permissiblethaw through the gravel.

The Finite Difference appraisals given in FIG. 14 are premised upon ayear l97l repair and modification of the existing storage unit. Allcurves in the figure are premised on air temperatures acting on thegravel up until September 1. It is assumed as of that date, the tank andthe insulation will begin to naturally cool under the influence of theambient thermal condition (i.e., to refreeze). With the exception of the6 inch insulation case, it will be understood that filling" (whenreservoir service begins or S-Day plus zero) will take place thefollowing January 1, thus allowing for a refreeze period. Analysis ofthe 6 inch case is based upon an earlier (October l) filling. FIG. 16shows how, after 20 years the mean Annual Soil Temperature (MAST) at adepth of 50 feet beneath the center of the tank is raised by insulation(e.g., as much as F).

In summary, the foregoing curves clearly demonstate that for such tankfacilities (including modifications), the following effects may bepredicted, assuming the described conditions:

1. If no insulation is utilized: the reservoir (tank) will have to beleft empty for about 3 months each winter to permit refreeze.

2. If 2 inches of ARCOFOAM are utilized: the reservoir may bemaintained, full of water, for about 4 to 5 years before it must beemptied and cooled for about 3 months (of normal winter) to effectrefreeze.

3. If 4 inches of ARCOFOAM are utilized: the reservoir may becontinuously kept full of water about 7 to 9 years before it must berefrozen" i.e., empty for about 4 to 6 months of normal winter.

4. If 6 inches of ARCOFOAM are utilized: the reservoir may be kept fullof water for approximately 12 to 14 years before refreeze;" for which itwill be left empty for about 9 to 12 months for normal winter.

5. It also appears reasonable to assume that if on the order of 10 to 12inches of ARCOFOAM were utilized, one would derive on the order of 20continuous years of reservoir operation before thaw would have traveleddown all the way through the gravel; however, the associated refreezetime could be upwards of 2 to 3 years a prohibitively long time for mostcases.

Here, it is surprizing to note the extent to which thicker insulationswill heat up sub-surface strata at considerable depths; e.g., the 10rise 50 feet down after 20 years (FIG. 15). One may conclude that, giventhe considerable heat-input which effects such a substrate heating oversuch a long period, a correspondingly long cooling period will berequired to dissipate its effects the inferance being that a veryconsiderable thermal inertia is provided by such frozen substrata.

EXAMPLE 5 To appraise the time requirement for refreeze in the 2 inchinsulation case, two analyses were run for a 5-year test period (seeFIG. 16) assuming a gravel embankment having the same soil properties asthe embankment of Example 1 (see FIG. 4). The first analysis run was fora zero insulation" case to establish a baseline or normal temperatureregime after 5 years. The second analysis was run for the same operatingstructure and conditions, however, assuming 2 inches of insulation inplace beneath the tank. At the end of 5 years, the tank was assumed tobe emptied and left in place over the 2 inch insulation. The solid-linewhiplash curves of FIG. 16 indicate that after a cooling of about 3 to 4months (October 1 to January 1), the subsurface heat-budget wasreasonably well restored (e.g., vs. a normal, empty pad of the same typeof dotted-line curves from October 28 to December 3 l In recapitulation,the foregoing description and features of novelty indicate someconclusions regarding service time and refreeze:

l. The existing tank of Example I (if repaired and intake-outletmodifications made) can be operated better if a refreeze period" isscheduled; with the original thermal regime restored by leaving the tankempty under ambient cooling for 3 months each winter (while also free ofsnow, preferably). With this modified operation and assuming arelatively normal year (e.g., no extraordinarily high-thaw-index orextraordinarily low freezeindex), the thaw periods (summer time) shouldnot cause significant permafrost melt or embankment subsidence.

II. The tank installation should be filled as early as possible beforelate spring (thaw) to minimize subsurface melt in a summer. This willhold true whether the tank is insulated or not.

III. 2 inches (effective) of ARCOFOAM, or equivalentinsa at e p d aeatLhqtank ndatetta.-.

winter weather). After this 5 .year service, the tank I EXAMPLE 6 As aresult of the foregoing analysis and explanation, it will becomeapparent that one can, using the features of novelty of the subjectinvention, develop a technique for composite foundation pad design andfor determining insulation thickness as a function of optimum servicetime and refreeze time for a given heatemitting facility to be supportedon the pad. Accordingly, one may determine an optimum insulationthickness as a function of the contemplated service (melting) andrefreeze periods assuming the installation constraints and theenvironmental factors indicated above for Examples 1 and 2; alsoassuming that service is initiated at the onset of the Arctic winter(about October l and, further assuming that the installation will beremoved from service during a prescribed optimum refreeze period.

Attention is next directed to FIG. 17, where thaw depth is plottedversus service time (solid lines only) for the subject heated-tankinstallation of Example I using various thickness of ARCOFOAMinsulation. FIG. 17 also shows a related plot of insulation thicknessversus refreeze time (dotted-line curve Y-R). In both cases the meltingtime and refreeze time will be understood as referenced to and basedupon maintainance of the (top of the) underlying permafrost formation ina frozen state (that is, the bottom of the foot gravel layer) and datumREF of FIG. 1 should never rise above 32F.

('71 difference) A tabulation comparing these thaw and refreeze curvesindicates that, since a typical arctic winter (e.g., at Site No. 1)gives only about 8 (:1) full, normal winter months of useful refreeze(approximating the conditions assumed in curve Y-R although FreezeIndex" is a better measure), one cannot use more than about 4-6 inchARCOFOAM insulation here without taking the subject installation out ofservice for about 2 full winters (possibly more) an extreme penalty!Note also that, compared with zero insulation, the incremental cost inadded refreeze time is only about 1 month with 2 inches of ARCOFOAM (vs.about 3 added months with 4 inches ARCOFOAM). Now, the 5-month totalrefreeze time with 4- inches is rather marginal, and might not besatisfied in one rather warm winter. Accordingly, the 2 inch foamthickness is preferred here (in related cases4 inches may instead bepreferable). With 2 inches, the extra expense (vs. using no foam at all)involved in accepting one extra month or refreeze time and in applyingthe 2 inches of AR- COFOAM should be weighed against the value of theadded service time (2.5 years) this produces. Similarly, a second 2inches of ARCOFOAM (the 4 inch case) plus the extra 2 months refreezetime it costs can be weighed against the value of extra serice time (2.5years) it provides.

Comparing these cost/benefit considerations with current economicfactors, it is usually most advantageous to use at least 2 inches ofARCOFOAM (or equivalent) insulation. Also, in light of theaforementioned desirability of beginning service approximately inOctober and beginning refreeze in January, it will be apparent that foaminsulation on the order of 2 inches is a very practical working solutionother cases are not so well suited to this schedule. For instance, ifservice had begun on Oct. 1, 1970 using the 2 inch foam insulation, onewould expect to take the installation out of service about Jan. 30, 1975(4.3 years later), and then begin a three-month refreeze, ending aboutApr. 1, 1975. The second service cycle could then begin (during Apr. of1975) and extend for a similar 4.3 years, ending in the fall of 1979, atwhich time a second refreeze period could begin etc., etc.).

Workers in the art will, of course, appreciate that the foregoingconditions and associated refreeze times and service times are to betaken as a statistical approximation and will vary with changes ingravel moisture, in weather at the site (e.g., abnormal F l or T1 forsite) and other conditions. They are given primarily for propaeduticreasons and serve to illustrate how one can employ certain featurestaught. For instance, although the time of service is given in years,for simplicity, it is as workers will understand) more properly afunction of the total degree days of thaw (thaw index T1) for a givenseason at the site and this can, of course, vary widely. Of course, thecumulative TI is a linear function with time because the facility issubject to a constant heat source. Similarly, although the refreeze timeis given about in months, it should more properly be expressed as afunction of total nominal degree days of freeze (Freeze Index Fl). Also,if one assumes that a selected refreeze period extends into a thawperiod, then a net (effective) F1, or the Total F1, less TI, shouldproperly speaking be determined and used. The thermal driving forcesinfluencing refreeze time are dependent upon climactic conditions which,of course, vary cyclically (e.g., diurnal temperature fluctuations) asopposed to a constant heat source. Therefore, the total cumulativedegree days of refreeze is going to be a nonlinear function with time,and it will vary with the time of year as well as with annualtemperature variations from the selected basis. Therefore, the period of3 months nominal refreeze time given for the 2 inches of ARCOFOAM case,must be understood as primarily illustrative of a January to Aprilperiod during thermal conditions that approximate those used as a basisfor the site. Accordingly, it will be evident that any given refreezetime may have similar deviations depending upon when (i.e., what seasonof the year) refreeze is initiated and upon what deviations in ambienttemperature conditions should be expected from the basis used.

EXAMPLE 7 The arrangement of Examples 1 and 2 is modified by providing asecond tank facility identical to that aforedescribed and constructed toalso supply heated water to the same use-station except, of course, thatconduits (inlet/outlet) and associated facilities are appropriatelymodified (e.g., duplicated). This second facility will be arranged andcontrolled (by conventional means none of this being illustrated) inconjunction with the first so that one tank may be selectively drawnupon(i.e., in service) when the other is taken out of service (e.g., duringits refreeze period) and so avoid interrupting the supply of water tothe common usestation. Preferably, this multible reservoir arrangementinvolves two or more tanks, each situated upon its own embankment pad,this being constructed according to the embodiment of Example 1 using 2inch AR- COFOAM insulation over 5 feet of gravel, with appropriateinterconnections for exchange of liquid between tanks, etc. Preferably,this second tank facility will be situated a prescribed minimum distanceaway from the first, at least enough so that any thaw-bulb developedunder one pad would be substantially unaffected by the adjacent pad. Aminimum spacing of the order of several dozen yards will suffice in thesubject case. Of course, third, fourth, etc. duplicate facilities maylikewise be provided to be alternatively used and cooled in the samemanner.

Workers in the art will recognize advantages derived from the foregoingdual (or multiple) systems; for instance, refreeze time may now beinvoked in the optimum, coldest season (e.g., beginning in January forsites 1 or No. 2), and maintained for a longer time, without concernover interrupting the liquid supply to usestations. (e.g., see Table[I]; if four 2 inch" facilities are provided, only one need be coolingat one given time).

EXAMPLE 8 The arrangement of Examples 1 and 2 is constructed with 2 inchARCOFOAM as before, except that, here, cooling-shunt means areinterjected above the 5-foot gravel layer to cover it entirely, thusbeing disposed below the insulation (i.e., just below layer T-i in FlG.1). An example of such a shunt is described in copending,commonly-assigned US. Pat. application Ser. No. 207,379 filed Dec. 13,1971 now US. Pat. No. 3,791,443 Richard Odsather, Kay E. Eliason and Albert C. Condo, and entitled Foundation for Construction on FrozenSubstrata. This will form, in effect, a rectitherm" embankment system asexplained below.

As a further modification of the foregoing cooling shunt construction,insulation layer T-i is modified to comprise a pair of like l2 inchARCOFOAM layers separated by spacer means adapted to introduce an airspace therebetween. Preferably this is accomplished by supporting eachfoam layer on a rigid structural platform (wood is satisfactory forordinary tank loading), the pair of platforms being spaced apart atleast a few inches by structural spacer members thereby form a doubleplatform supporting the superposed tank structure, this being entirelyencapsulated in insulation with l to 2 inch ARCOFOAM extending entirelyacross each platform and covering all side areas extending between theplatforms as well. Preferably, the sides defining this inter-platformair space are covered with foam blankets and preferably portions ofthese blankets are rendered removable or displaceable at opposite sidesof the air space to allow selective introduction of ambient aircross-circulation (when the blanket sections are so displaced). Thesides are preferably insulated in the same manner as the platforms withl to 2 inches of ACROFOAM on suitable supporting means. Moreparticularly, it is preferred that hatches or swinging circulationports, are provided for this purpose, being adapted to be swung open forcold air circulation during cooling weather, such circulation intended,of course, to accelerate dissipation of heat from the gravel andpermafrost beneath. Such an expedient will thermally shunt the overlyinginsulation layer and tank facility, allowing heat escaping therefrom tobypass the thermal impedance associated therewith. The hatches will beclosed during warm (thaw) weather to thermally isolate (stagnate) theairspace and, taking advantage of its insulating properties, helping toimpede heat input to the substratum.

Workers in the art will appreciate many advantages to this feature. Forinstance, such a cooling-shunt means will be appreciated as providing anew and useful system for helping to retard heat-input to a permafrostsubstrate (e.g., from a heated structure on an embankment pad; as wellas from warm ambient air) as well as to, selectively (e.g., during coldweather), accelerate heat-output from this substrate; e.g., as an aid tominimize refreeze time. Stated otherwise, the system can provideimproved insulation during warm weather together with improved heatdissipation during cold weather. Such a system could thus becharacterized as a unidirectional or assymetric conductor (or heat i.e.,a one-way heat valve); or, to analogize to electrical conductors (cf.rectifiers which are unidirectional conductors of electric current), arectitherm system as it were. However, without the use of suchrectitherm means, most users of the subject embankment constructionfeatures will tend to derive such rectitherm effects anyway, using otherexpedients. For instance, the aforedescribed technique of employingminimal insulation thickness and have maximum winter cooling" whilestill providing an acceptable mean thaw during warm weather itselfexhibits a certain rectitherm effect. That is, if only a mean thicknessof insulation is used, such as to barely provide the least acceptableprotection against thaw, as has been explained, this will optimize thecooling and restoration of the embankment heat budget during coolweather. According to this teaching of minimal insulation techniques, itis critical, yet not obvious, that the greater the insulation thickness,the more one will impede substrate cooling (e.g., dissipation during aprescribed refreeze period of geothermal and other heat taken up bypermafrost which can, over an extended period, degrade the usefulness ofthe facility supported). This is a rather surprising teaching, contraryto the heretofore accepted NORM: that is, workers have to the presentaccepted that the thicker the insulation, the better" (aside fromincreased material costs, of course).

EXAMPLE 9 The arrangement of Example 8 is constructed as above exceptthat phase conversion means is incorporated into the embankment systemto enhance heat dissipation. More particularly, the air-space betweenthe described insulated platforms is employed to receive a resilientpillow tank of inflatable plastic (or the like) and this tank is partlyfilled with freezable phase-conversion means in the form of water,leaving only sufficient space therein to allow for freezeexpansionwithout rupture. The water will freeze during typical cold weatherservice conditions and, as ice, will drastically retard the improvementof any thawfront therepassed. Workers in the art will readily recognizethat a substantial ice mass likethis will present enormous thermalimpedients to passage of a thawfront in that the front must first giveup enormous quantities of heat (latent heat of fusion) and convert allice to water before it passes beyond the mass. Preferably, thispillow-tank will extend over and intersect substantially all of thecross-section of the air-space (but only part of its height) so as tointerrupt all heat flow through the pad. This modification may be usedsupplementarily with the above-described icing-in of the subjacentgravel or as a substitute therefor. Obviously, unlike icing-in," it willinvolve no risk of liquid loss, run-off or resulting subsidence.

Of course, workers in the art will perceive equivalent phase conversionmeans (or fusible fillers") to use instead of water, such as an aqueousglycol solution, brine (e.g., using sea water), etc.; or other liquidsthat will freeze at the ambient service temperatures, expected and willhave a reasonably large, useful latent heat of fusion (preferablycomparable to water or better). Of course, if such a filler has a highermelting temperature (as solidified) than water, it will offer even moreprotection for the permafrost substrate since it will melt much sooner.

Workers will also contemplate other analogous container means for suchfusible fillers. For instance, plastic (vinyl chloride polymers) bagswill be feasible for certain applications; an example of such a baggiven in U.S. Pat. Nos. 3,381,441 to Condo et al., 3,501,433 to Condoand 3,419,511 to Condo et al. Such bags are relatively small,inexpensive and readily available and this will often provide a superiorcontainer module (or water bag") adaptable to a wide variety of fusiblefiller applications for instance offering a volumemodule adapted foroccupying a wide variety of spaces (size, shape). Similar bags may bemade of like elastomeric material preferably being resilient enough toaccommodate freeze-expansion. Likewise, certain rigid containers may beemployed, such as tin cans, fibre foil containers, glass or plasticbottles, oil drums or the like as long as they are adapted toaccommodate the freeze-expansion of the liquid fill (e.g., by leavingadequate expansion space therein) and, of course, are arranged so as toeffectively retain the liquid (e.g., by sealing the drums, capping thebottles or cans, etc.). Employment of such containers would not onlysolve a disposal problem but provide containers which are longer-livedand more stable (e.g., resistant to corrosion, leakage) than theaforedescribedpillow-tank or plastic bags. A related advantage is thatsuch containers may be second-hand, somewhat dirty, etc. and thusinexpensive; their use may also help alleviate waste disposal problems(e.g., used oil drums, discarded bottles, cans).

The phase-conversion containers may, in many instances, be otherwisehoused also. For example, as opposed to placing the describedpillow-tank inside the described inter-platform airspace (which, ofcourse, relieves the tank of any top-loading from the facility above andeliminates the risk of pillow-tank rupture, leakage and resultantembankment subsidence), the circumstances of service may permit directstructural coupling of the pillow to the embankment and superstructure,so long as the slight rise and fall of the tank (with freeze/thaw) andof the entire embankment and facility it supports can be tolerated inwhich case the platforms wouldbe eliminated with the pillow-tankreplacing (or supplementing?) the entire cooling shunt structure. Here,the risk of rupture is, of course, accepted; however, if the full-tankthickness is only an inch or so (e.g., and a freeze-plug is used), therisk may be tolerable --especially where a plurality of such thinpillow-tanks is piled-up to provide the overall liquid volumecontemplated. However, direct loading of smaller resilient containerslike the recited bags will be less desirable since it will present theadded risk of differential settlementHowever, the described rigidcontainers may also be loaded directly, such as by burying sealed oildrums (partly-filled with water) in an embankment. Of course, the smallplastic bags (or like resilient containers) may be housed in othervarious facility-supporting structures; for instance, the plastic bagsmay be nested within a honeycomb matrix of structural cellular plastic(e.g., rigid urethane, styrene foam or water-resistant fibre) fashionedto receive them and able to support the top-loading structures. Moreparticularly, the bottom of the water-storage tank T-W of FIG. 1 may beprovided'with a base of rigid urethane foam (under the vinyl liner Tv)comprising a solid flat urethane sheet with bag-receiving pockets on theunderside thereof and the recited water-bags nested therein.Alternatively, a simple metal grid (open-mesh surrounding bagsconductively while supporting a continuous-sheet upper load such as asheet of rigid urethane) may be used under a rigid support means. Otherforms of such phase-conversion means and accessories therefore will becontemplated.

EXAMPLE 10 The arrangement of Examples 1 to ,2 is constructed asdescribed except that, while the gravel pad is being laid, it is sowetted with water and so compacted (or kept compacted with a minimumpercent void) as to coat and wet at least a substantial percentage ofthe gravel particles with ice around the surface thereof, and so as toalso fill a substantial portion of the interstitial spaces therebetweenand thereby form a composite ice-particulate pad (assuming ambientconditions adequate to freeze the water film). This wetting must,however, not substantially swell" the gravel layer and thus will notsupply sufficient excess interstitial liquid to substantially move orseparate the compacted particles mechanically the process thusconstituting a stable wetting technique, which, after j freezing andlater thaw, will not yield any detrimental lift or subsidence effects.Thus, the particle size, and degree of compaction must be kept withinlimits (high effective-density; with low percent-void) to provideadequate stability and liquid-retention capacity to the compacted-gravelpad (at least until freeze-up sets in). This is generally determinableaccording to the amount of the (initially) applied water retained inthegravel mass, after a certain time to allow for run-off of free,non-adsorbed liquid. Compaction (overall pad density or percent void)will not be so high as to prevent liquid from effectively percolatingthrough the particle interstices and from wettingmost of the particlesurfaces. Once some, or all, of the 5-foot gravel pad is so wetted andthen freezes (the particles thus being coated with a thin film of ice orrimed) with insulation superposed, the tank facility being placedthereon, etc.), it will be evident that a substantial heatabsorbingcapacity is added to the foundation pad whereby, as the thaw-frontproceeds down through the gravel sorimed", it encounters a thermalimpedance which is greatly amplified, since it must convert ice towater, replace the latent heat of fusion as it proceeds, and itsprogress is accordingly very greatly retarded. The result, of course, isa much extended service time; often enough to dispense with any need torefreeze. Of course, the fusible filler (where employed) will be meltedthewhile and subject to loss (run-off, evaporation, etc.) unlesspackaged as above indicated. Thus, it will often be preferred to avoidany such packaged phase-conversion means and use only the describedriming technique for introducing ice as a thaw front barrier (thermalimpedance).

The enhancement of the warm-weather stability of embankments constructedas described will obviously be enormous though surprising in light ofthe simple means used. Moreover, workers will appreciate that, using therimed-gravel technique, (even aside from whether the wetting" moistureconverts to ice) will provide a new and eminently useful embankment(gravel) material which has a prescribed, predictable homogeneousmoisture content (a wetted-gravel). Using such can render particulatepads unlike any heretofore used in such circumstances, being not onlypremoistened, but moistened to an unusually uniform degree to thusimpart much more uniform, predictable thermal properties (e.g., comparea homogeneous moisture content of about 14 percent throughout thethree-dimensions of gravel bank P-B in FIG. 1 with one which exhibitsthe indicated typical ranges of 5-14 percent moisture as in FIG. 4).

The attendant advantages are tremendous. Now, a pad designer can freehimself from concern over varying moisture content when using virtuallyany kind of native gravel (e.g., he can ignore the moisture factors ofthe gravel regardless of source or its shelftime and associatedevaporative losses), and is liberated from the consequent variations inreliability and stability. Instead, he can calculate from a constantreliable reference moisture as though the pad gravel were artificiallymanufactured to his specifications.

Workers will also perceive various refinements of this wettedembankment" construction. For instance, in some cases, a suitableinterstitial moisture-absorber such as sawdust. may also be incorporatedwith the gravel particles. Moreover, in some cases, the wetting may berenewed after melting (end-of-service). This will at times be effectedby removing overlying structures and re-moistening the gravel to theproper wetness, where feasible; or by incorporating a moisture-deliveryduct system throughout the gravel thickness and introducing moisturethrough this at appropriate times. One may even employ an airduct systemof the type described in the aforementioned Application where feasible,enabling the ducts thereof to perform a re-moistening" function inaddition to circulating coolant air. Also, in cases where gravel orother soil particulates are not desired as embankment material, othermaterial may be substituted in certain cases, such as perforate mats ofplastic webs, chipped nondegradable fines or other materials adopted toexhibit the necessary structural qualities and the designatedmoisture-penetration.

Likewise, the described phase conversion means may be employed, and/ormodified, with or without the other described features or, in other,equivalent ways. Also, for cold weather facilities like those describedit will be apparent that workers may, according to this teaching, employother thermal impedance means and- /or heat dissipation means to yieldthe described rectitherm effect, for instance, using just sufficientinsulation to keep the frozen substrate from melting and NO MORE, lestthe cooling thereof be impaired; and/or cooling-shunt means for thispurpose; and/or the described Finite Differencing Technique for improvedembankment design; and/or insulated embankment, the construction andmaterial, of which are designated as a function of service life, siteambient temperature history (especially deviations from the norm" forthe site), facility temperature and heat-input, geothermal heat, soilcomposition (especially moisture content), subsurface soil temperatureprofiles, or a combination of these, as well as other factors such asthe initial thermal regime; and/or the cooling (refreeze) mode for thegiven embankment at the given site; and/or the provision of aparticulate soil transition-zone under the facility designed toaccommodatea prescribed thawtransit (i.e., travel of thaw-locus asdriven by environment heat-input); and/or the provision of artificial(e.g., urethane foam) insulation above this transitionzone; and/or thescheduling of prescribed times for service initiation or for refreeze ora prescribed insulation thickness corresponding thereto for the givenfacility and service conditions; and/or the provision, otherwise, of anartificial active layer (or pseudo-turf) beneath such facilities, and/orthe provision of an ice layer thereunder; and/or the alternation of aplurality of such facilities to take at least one thereof out of servicefor such cooling. In particular, workers may, according to featurestaught, employ a correlation of surface and subsurface temperatures forthe site (soil) with conventional thermal data, for extended timeperiods (function of service life contemplated) to determine the amountof insulation (as optimized); and/or to determine the likely servicetime and/or refreeze time, this being recognized as widely advantageous,especially for a cost/benefit comparison of insulation thicknesses.

The foregoing features of invention will be understood as described onlyin exemplary emobidments and obviously applicable with other equivalentmeans and for analogous purposes, the scope of protection pertaininghereto being limited only by the appended claims. This is, it is obviousthat various modifications of the structures and/or techniques taughtherein may be made without departing from the spirit of the invention asdefined in the appended claims. For example, equivalent elements andsteps may be substituted for those described, parts may be reversed andvarious features may be used independently of other features, allwithout departing from the spirit of the invention.

What is claimed is:

1. A method which permits the long term continuous use of a heatedliquid storage reservoir in permafrost regions said reservoir beingsupported by frozen terrain and being thermally insulated from saidterrain by a thermal barrier comprised of a layer of synthetic thermalinsulation, said method comprising the steps of:

a. Storing heated liquid in the reservoir until the thaw front resultingfrom the transmission of heat from the heated liquid downwardly throughthe frozen terrain extends to a predetermined depth,

b. Removing substantially all of the heated liquid from the reservoir,and

c. Permitting the terrain beneath the reservoir to refreeze.

2. The method of claim 1 wherein the layer of synthetic thermalinsulation is sufficiently thick to permit the reservoir to becontinuously used for several years without the necessity of refreezingthe underlying terram.

3. The method of claim 1 wherein the heated liquid is water.

4. The method of claim 1 wherein said thermal barrier includes a layerof gravel.

5. The method of claim 4 wherein the constitution and thickness of saidthermal barrier is such as to provide a reservoir use period of 4 to 9years and a refreeze period of 3 to 6 winter months.

6. The method of claim 4 wherein said gravel layer is 4 to 6 feet thick.

7. The method of claim 6 wherein said layer of synthetic insulation is 2to 4 inches thick.

8. The method of claim 1 wherein said synthetic thermal insulation isfoamed polyurethane.

9. The method of claim 1 wherein two or more storage reservoirs areemployed together and cycled such that at least one reservoir is in usewhen another is out of service for refreezing the terrain lyingthereunder.

10. The method of claim 1 wherein said thermal barrier includes a heatdissipating means.

11. The method of claim 10 wherein said heat dissipating means comprisesa cooling shunt means.

12. A method which permits the long term continuous use of a heatemitting liquid storage system comprised of a plurality of liquidstorage reservoirs supported upon frozen terrain in polar regionswithout excessively melting and thereby destroying the support for thereservoirs, said reservoirs being thermally insulated from said frozenterrain by a thermal barrier comprised of a layer of gravel and a layerof synthetic thermal insulation, said method comprising using eachreservoir on a given cycle comprised of an in service period and arefreeze period the cycles being such that when one of the reservoirs ison the refreeze period another is on the in service period so that thesystem as a whole providescontinuous service.

1. A method which permits the long term continuous use of a heatedliquid storage reservoir in permafrost regions said reservoir beingsupported by frozen terrain and being thermally insulated from saidterrain by a thermal barrier comprised of a layer of synthetic thermalinsulation, said method comprising the steps of: a. Storing heatedliquid in the reservoir until the thaw front resulting from thetransmission of heat from the heated liquid downwardly through thefrozen terrain extends to a predetermined depth, b. Removingsubstantially all of the heated liquid from the reservoir, and c.Permitting the terrain beneath the reservoir to refreeze.
 2. The methodof claim 1 wherein the layer of synthetic thermal insulation issufficiently thick to permit the reservoir to be continuously used forseveral years without the necessity of refreezing the underlyingterrain.
 3. The method of claim 1 wherein the heated liquid is water. 4.The method of claim 1 wherein said thermal barrier includes a layer ofgravel.
 5. The method of claim 4 wherein the constitution and thicknessof said thermal barrier is such as to provide a reservoir use period of4 to 9 years and a refreeze period of 3 to 6 winter months.
 6. Themethod of claim 4 wherein said gravel layer is 4 to 6 feet thick.
 7. Themethod of claim 6 wherein said layer of synthetic insulation is 2 to 4inches thick.
 8. The method of claim 1 wherein said synthetic thermalinsulation is foamed polyurethane.
 9. The method of claim 1 wherein twoor more storage reservoirs are employed together and cycled such that atleast one reservoir is in use when another is out of service forrefreezing the terrain lying thereunder.
 10. The method of claim 1wherein said thermal barrier includes a heat dissipating means.
 11. Themethod of claim 10 wherein said heat dissipating means comprises acooling shunt means.
 12. A method which permits the long term continuoususe of a heat emitting liquid storage system comprised of a plurality ofliquid storage reservoirs supported upon frozen terrain in polar regionswithout excessively melting and thereby destroying the support for thereservoirs, said reservoirs being thermally insulated from said frozenterrain by a thermal barrier comprised of a layer of gravel and a layerof synthetic thermal insulation, said method comprising using eachreservoir on a given cycle comprised of an in service period and arefreeze period the cycles being such that when one of the reservoirs ison the refreeze period another is on the in service period so that thesystem as a whole provides continuous service.